linux-kernel - Re: [PATCH 6/6] lib: Add function strscpy_from

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Date:   Tue, 19 Feb 2019 03:12:33 +0100
From:   Jann Horn <jannh@...gle.com>
To:     "Tobin C. Harding" <tobin@...nel.org>
Cc:     Kees Cook <keescook@...omium.org>, Shuah Khan <shuah@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] lib: Add function strscpy_from_user()

On Tue, Feb 19, 2019 at 12:25 AM Tobin C. Harding <tobin@...nel.org> wrote:
> Currently we have strncpy_from_userspace().  If the user string is
> longer than the destination kernel buffer we get an error code -EFAULT.
> We are unable to recover from here because this is the same error
> returned if the access to userspace fails totally.
>
> There is no reason we cannot continue execution with the user string
> truncated.
>
> Add a function strscpy_from_user() that guarantees the string written is
> null-terminated.  If user string is longer than destination buffer
> truncates the string.  Returns the number of characters written
> excluding the null-terminator.
>
> Signed-off-by: Tobin C. Harding <tobin@...nel.org>
> ---
>  lib/strncpy_from_user.c | 43 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 43 insertions(+)
>
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index 11fe9a4a00fd..6bd603ccec7a 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c

This file is only built when CONFIG_GENERIC_STRNCPY_FROM_USER is set.
Some architectures have their own versions of strncpy_from_user() and
don't set that, so on those architectures, your code wouldn't be built
into the kernel.